1. Field of the Invention
The present invention relates to a new and improved method of calendering a material web, in particular, a paper or cardboard web preferably, although not necessarily coated at both sides or faces thereof.
In its more specific aspects, there is disclosed a method of calendering, that is to say, smoothing and glazing a material web, in particular, a paper or cardboard web which is preferably, although not necessarily coated at both sides or faces within at least two treatment nips or gaps arranged in succession with respect to a predetermined direction of travel of the material web. This material web has imparted to both of its web sides or faces a desired smoothing effect or smoothness and a desired glaze or gloss.
2. Discussion of the Background and Material Information
The calendering or calender treatment usually constitutes the last process stage during the manufacture of paper or cardboard. For this purpose there are employed apparatuses such as, for example, so-called calender installations or supercalenders. They serve for the calender treatment of the paper or cardboard web at both sides or also even at one side thereof. There are available calender installations having so-called "hard" and "soft" treatment nips or gaps. In the case of calender installations containing the hard nips, the surfaces which bound or delimit both sides of the treatment nip are practically non-yieldably hard, and for those calender installations containing soft nips there is usually employed a yieldable elastic surface at least at one side of the treatment nip. The hard surfaces are frequently heated.
The commercially available calender of the assignee of the present invention, sold under the trademark "NipcoMat", constitutes a calender having soft treatment nips. This calender can be provided with one or a number of treatment nips. The surfaces bounding the treatment nips are typically formed by rolls or cylinders having hard metal roll or cylinder surfaces or elastically yieldable coated roll or cylinder surfaces.
There have also been proposed calender installations or calenders where the treatment nip is formed between two revolving bands or belts. Significant in this regard is the commonly assigned German Patent Publication No. 3,920,204, published May 10, 1990. The dual-sided calendering of paper or cardboard coated at both sides or faces usually is accomplished in at least two successively arranged soft treatment nips, and each web side or face is processed at both hard and also elastically soft rolls. When operating with only two treatment nips, then, in the first treatment nip the one side or face of the material web comes into contact with a hard roll and the opposite side or face with a soft roll. In the second treatment nip, the web side or face which was previously processed by the hard roll now comes into contact with a soft roll, and conversely, the other web side or face, which was previously in contact with a soft roll, now comes into contact with a hard roll. A drawback which has been found to exist with this calendering method, particularly when processing high-glaze types of paper or cardboard, resides in the fact that the high degree of glaze or gloss which has been attained at the one web side or face due to web contact with the hot, hard surfaces of the heated rolls, is again diminished in the subsequent treatment nip when the web comes into contact with rolls having yieldable elastic surfaces.